Research has been carried out on the boring and processing of rock with a laser (see, for example, Patent Document 1).
When rock is continuously irradiated with a laser, the rock melts and dross is generated, causing a problem that the dross hinders processing the rock with the laser. In the above technique, to prevent dross from being built up and to thereby avoid hindering the progress in processing the rock, assist gas is blown onto a laser irradiated part to encourage discharge of the dross.
Moreover, when being rapidly and locally heated, some kinds of rock are easily destroyed and other kinds of rock are hardly destroyed, depending on various factors such as the kind, constituents and water content (see, for example, Non-patent Documents 1 and 2).
There is also a processing technique for, for example, processing rock by irradiating the rock with a laser while preventing the deposition of dross by use of fracture characteristics of the rock (see, for example, Patent Document 2).
In this technique, local rapid heating fracture characteristics of rock targeted for working are investigated, and laser intensity and interaction time between the laser and the rock are adjusted on the basis of the investigation, to irradiate the rock with a laser under such irradiation conditions that no dross would be deposited. This technique does not require: addition of any additive agent for increasing the flowability of dross; blowing of assist gas; or the like. Accordingly, the processing of rock can be remotely performed. However, it is difficult to process rock having inappropriate fracture characteristics induced by a laser, such as rock having a high silicon dioxide content, for example.
Further, there is also a technique for excavating a stratum such as an underground resource containing layer in liquid with a laser (see, for example, Patent Document 3).
In this technique, by the induction force of a first laser and the thermal action of a second laser transmitted through bubbles, the stratum in the liquid is excavated. The induction force of the first laser is acquired by an action of a shock wave, a jet flow, a bubble flow or a sound wave, or an action based on a combination of two or more of these phenomena. As the first laser, a pulse laser or a continuous wave laser emitted intermittently is used.
Moreover, it is known that the absorption factor of a laser at which the laser is absorbed into liquid varies depending on the wavelength of the laser, and also that the absorption characteristics of a laser vary depending on the wavelength (see, for example, Non-patent Document 3). A laser having a longer wavelength has a higher factor of absorption into liquid.
Meanwhile, fibers capable of transmitting a laser having a long-wavelength range have emerged.
For example, for a wavelength range in which the wavelength of a laser is no less than 2.0 μm, infrared-transmitting fibers (IR-transmitting fibers) are used. IR-transmitting fibers are optical fibers capable of transmitting infrared light having a wavelength longer than the transmission limit of silica optical fibers (approximately 2 μm). In the case of using a Ho-YAG laser (approximately 2 μm wavelength), a fiber having SiO2 or GeO2 as a core material is used. In the case of using an Er-YAG laser (approximately 3 μm wavelength), a fiber having fluoride glass as a core material is used. In the case of using a CO laser (approximately 5.3 μm wavelength) or a CO2 laser (approximately 10 μm), a fiber having chalcogenide glass as a core material can be used (see, for example, Non-patent Document 4). Alternatively, hollow fibers are also used (see, for example, Non-patent Document 5).    Patent Document 1: Japanese Laid-open Patent Publication No. 2001-170925 (pp. 2-3, FIG. 1)    Patent Document 2: Japanese Laid-open Patent No 2006-305803 (pp. 3-4, FIG. 1)    Patent Document 3: Japanese Patent No. 3856811 (pp. 4-5, FIG. 1)    Non-patent Document 1: B. C. Gahan et al., “Laser drilling: determination of energy required to remove rock,” SPE 71466, October 2001 issue    Non-patent Document 2: Institute of Electrical Engineers of Japan, investigation committee of laser ablation and its industrial application, ed., “Laser ablation and its industrial application,” November 1999 issue, Corona Publishing Co., Ltd., pp. 1-9, FIG. 1.1    Non-patent Document 3: Laser Heat Processing Society, ed., 39th laser heat processing society collected papers (November 1996), pp. 87-91    Non-patent Document 4: Laser Society of Japan, ed., Review of Laser Engineering, Vol. 27-3 (15 Mar. 1999 issue), pp. 167-172    Non-patent Document 5: Laser Society of Japan, ed., Review of Laser Engineering, Vol. 27-3 (15 Mar. 1999 issue), pp. 173-177
Rock often contains quartz glass or silicon dioxide. When such rock is irradiated with a laser, the quartz glass or silicon dioxide is deposited as molten dross due to the heat of the laser. Once molten dross is deposited, even if the rock is further irradiated with a laser to encourage melting, the laser is reflected and scattered, so that the amount of heat of the laser absorbed by the rock is insufficient For this reason, further progress in processing the rock is difficult to make. The dross melting during laser irradiation solidifies when the laser irradiation is stopped, and the borehole is filled with the dross.
The present invention provides a technique capable of processing rock even containing quartz glass or silicon dioxide which is deposited as molten dross by laser irradiation. In addition, in the present invention, a single laser, without using first and second lasers, is used for the processing of rock. An object of the present invention is to provide means for solving the problem that processing is hindered due to generation of molten dross.